Single face corrugated paperboard webs are made on a single facer machine. A liner is attached to the crests of the corrugations to form double face corrugated paperboard at a double facer glue machine. At the glue machine, the web passes between an adhesive applicator roll and a rider roll. The amount by which the rider roll compresses the corrugations establishes the amount of adhesive applied. Accordingly, the rider roll must be ballasted to apply either a constant predetermined pressure or to maintain a constant predetermined gap between its periphery and the adhesive applicator roll.
The effective pressure on the rider roll can be maintained either by a pressure control arrangement as described in U.S. Pat. No. 2,876,734 or by controlling the gap between the rider roll and the adhesive applicator roll. To maintain a uniform compression of the single faced paperboard as it passes between the rolls, the thickness of the web must be sensed. Thus, there is need for sensing apparatus capable of acting on the corrugated side of the web for measuring the thickness of the web as the web is moving at high speed so that appropriate adjustments and/or stopping of the web can be attained. The flexibility, the ease of deformation and the undulated nature of the corrugated side of the web complicate the problems in designing suitable sensing apparatus.
In addition to knowing the caliper of the single face web entering the glue machine to control the gap, it is necessary to position a rider roll at a preset distance or gap from the adhesive applicator roll and to have the rider roll vary this gap as the caliper of the web entering the glue machine increases or decreases. Thus, the single face flutes are always under a predetermined compression as they contact the applicator roll and consequently each flute tip picks up an equal amount of a bonding agent.
Improper compression of the single face flutes causes both wastage of bonding agent and excessive consumption of heat in the double facer. If the board is compressed too much, the caliper and strength may be too low for commercial use. Partially crushed board has reduced column strength and dimensioned compression resistance. On the other hand, insufficient application of adhesive lowers the quality of the corrugated board by reducing pin adhesion.
Accordingly there is a need for apparatus having automatic controls for applying the optimum amount of an adhesive bonding agent under changing conditions.